Use of dioxane as a solvent for vapor plating molybdenum, tungsten and chromium from their hexacarbonyls



United States Patent 3,023,491 USE OF DIOXANE AS A SULVENT FOR VAPORPLATING MGLYBDENUM, TUNGSTEN AND CHROMIUM FROM m IEXACONYLS Elmer RobertBreining and John R. Whitacre, Dayton,

Ohio, assignors, by mesne assignments, to Union Carbide Corporation, NewYork, N.Y., a corporation of New York Filed Jan. 2, 1958, Ser. No.706,843 3 Claims. (Cl. 29-194) This invention relates to coatings on thesurfaces of bodies or substrates; more particularly the inventionrelates to the deposition of a coating by the thermal decomposition of aheat decomposable compound in the presence of vapors of the compound andvapors of a solvent for the compound. Still more specifically theinvention relates to the deposition of a coating by the thermaldecomposition of compounds selected from the group of molybdenum,tungsten and chromium carbonyls.

The thermal decomposition of molybdenum carbonyl, which isrepresentative of the compounds under consideration, for example, hasbeen known; however, to attain adequate deposition within reasonableplating times it has been necessary to employ vacuum or materiallyreduced pressures. This is due to the fact that the vapor pressurenormally exerted by the molybdenum compounds is relatively low. Whilehereinafter molybdenum will be referred to in detail, it is to beunderstood that the same general characteristics and utilities apply tothe carbonyls of tungsten and chromium.

This present invention provides for the effecting of the decompositionof the molybdenum compound While it is in the vapor state and associatedwith vapors of a solvent for the molybdenum compound. Further, normallyassociated with the vapors of solvent and vapors of the molybdenumcomponent, in the practice of the invention, there is an entraining gas.Such combination provides for a relatively high concentration of themolybdenum compo nent in the vapor phase and relatively fast plating isachieved thereby.

It is therefore a principal object of this invention to provide a novelmethod for the deposition of coatings from thermally decomposable metalbearing compounds.

It is an important object of this invention to provide a novel methodfor the deposition of continuous, nonporous, adherent coatings onsubstrates of metals, including alloys such as stainless steel.

A particular object of the present invention is the provision of coatedsubstrates in which the coating is deposited thermally by a vapor phaseplated metal.

It has been found that molybdenum bearing compounds, such as molybdenumcarbonyl, which is normally solid at room temperature, may be dissolvedin cyclic ethers, for example, and that the molybdenum compound when theether is heated exerts a considerable vapor pressure. This is inconsiderable contrast to the vapor pressure of the carbonyls(hexacarbonyls) of molybdenum, tungsten and chromium at room temperaturewhich pressure is about 0.1 mm.

While plating with vapors from the solution in which the carbonylcomponent is dissolved is practical, it is preferable to increase thequantity of carbonyl in the vapor phase per unit of time by passing anentraining gas for the carbonyl component through the solution. Suchentraining gas may, for example, be nitrogen, argon, carbon dioxide, orany other gas which is inert or substantially inert to chemicalreaction, at temperatures up to the decomposition temperature of themetal carbonyls.

The ethers utilized in the practice of the invention should bemaintained free of water and oxidizing constituents in order to avoiddeterioration of the carbonyl com- 3,023,491 Patented Mar. 6, 1962pound. In the practice of the invention the ether which is preferablyemployed is 1,4-dioxane. The cyclic ether exhibits a boiling point of101 C. and when heated readily dissolves the carbonyls, which are whitesolid at normal temperatures and decompose thermally at about 150 C.Another cyclic compound found to be useful is tetrahydro furan; thisS-member ring compound has a boiling point of about 65-66 C. and likethe 6-membered ring compound dioxane is stable for the purpose of theinvention.

The solid carbonyl dissolves in the dioxane only sparingly at roomtemperature, but is quite soluble at temperatures approaching theboiling point of the dioxane. Accordingly it is preferred to use thedioxane while heated in order to establish an adequate vapor pressure ofthe carbonyl. Preferably also the molybdenum component, such as thecarbonyl, is provided in excess in contact with the solution in orderthat the solution will be maintained saturated throughout the procedure.

The coating procedure is accomplished by providing the object orsubstrate to be coated into the vapors containing the metal carbonylcomponent. The exposure to the vapors may be by a static method or bydrawing the substrate through the vapors of the metal carbonylcomponent. Normally the temperature of the solution with which thevapors are in contact is -100 C. and the substrate itself is heated toat least the decomposition temperature of the metal carbonyl component.In the case of molybdenum carbonyl, for example, the temperature shouldexceed C. and preferably is well above this decompositiontemperature-that is up to 315 C. (600 F.). Customarily operation in therange of ZOO-250 C. is satisfactory. The substrate temperature howeverwill vary with the nature of the substrate, but must be sufficientlyhigh such that it remains at plating temperature a sufficient length oftime to effect a deposit. Such minimum temperature is generally about150 C.. for a few seconds. Thicker deposits are attained by providingthe substrate at a higher temperature and for a somewhat longer periodof time in contact with the vapors. Thicknesses, however, may be builtup by repeated presentation of the substrate to the vapors. The natureof the substrate will, of course, affect the maximum temperature towhich the substrate may be heated; for example, substrates of mostmetals may be heated to 300 C., or more, While with many plastics aconsiderably lower temperature is necessary.

The invention will be more fully understood by reference to thefollowing detailed description and accompanying drawings wherein:

FIGURE 1 schematically illustrates an apparatus arrangernent for thepractice of the invention by a static method;

FIGURE 2 schematically illustrates an apparatus arrangement for thepractice of the method of invention as applied to wire, tape, andsimilarly configurated materials; and

FIGURE 3 is a cross-sectional view of a wire metallized with molybdenumin accordance With the invention.

Referring to the drawings, the numeral 1 in FIGURE 1 designatesgenerally apparatus useful in the practice of a static method for thesubstrate to be plated.

In FIGURE 1 a vessel 2 is provided with a closure 3 as of glass, forexample. Within the vessel and substantially half filling the same is asolution 4 of molybdenum carbonyl in 1,4-dioxane; solid molybdenumcarbonyl in contact with the solution being indicated at 5.

Through the closure 3 there is provided a stopper 6 of Teflon whichreceives a conduit 7 for the passage of the entraining gas to thesolution as indicated at 8. The conduit 7 dips well into the solution.The numeral 9 designates an exhaust conduit passing through the stopper10. Extending into the space 11 above the solution 4 is a substrate 12,such as a rod of stainless steel; this stainless steel rod is supportedin a stopper 13 which also supports a thermocouple, the leads of whichare indicated generally at 14. The rod is heated to about 277 C. (527F.).

If desired this substrate 12 may be heated electrically, for example,during the course of the plating operation. The solution 4 is itselfheated by an electric element, such as a hot plate, designated at 15 inFIGURE 1. Preferably the temperature of the solution is maintained atvery nearly the boiling point of the solution in order that the quantityof carbonyl in the vapor phase will be as high as practicable. However,the solution may be as low as 90 C. and still provide adequate platingwithin a reasonable time.

Also the flow of the entraining gas is in the present instance such thata voluminous bubbling of the gas through the solvent is apparent. 0.25cubic foot per minute is suflicient when about 50 grams of the solventare employed in a beaker.

Referring now to FIGURE 2, the numeral 16 indicates a resistance heaterof substantially cylindrical contour, through the center of which therepasses a copper wire '17, unrolled in any conventional manner from areel indicated at 18. The wire 17 in its traverse passes through a gasseal 19, chamber 20' and an outlet gas seal 21 to a. winding device 22.The chamber 20 is provided with an inlet port 23 and an exhaust port 24.The inlet port 23 is connected by a conduit 25 with a vaporizer 26,which itself consists of a tank 27 surrounded by a steam jacket 28. Thejacket 28 maintains the temperature of the solution of dioxane andmolybdenum carbonyl indicated at 29 to within the range of 90l00 C.

The entraining gas, which as previously indicated may be an inert gas,such as CO or nitrogen, is passed through a conduit 30 into the solution29 and the vapors exit through the conduit 25 to the chamber 20. In thechamber 20 the vapors contact the wire 17, which has been heated to atemperature of 2SO275 C., and the product of decomposition passesthrough exhaust 24 to any suitable exhaust arrangement.

Referring now to FIGURE 3, the numeral 17 designates a section of copperwire and the numeral 18a indicates a hard coating of the metallicconstitutent of the carbonyl completely surrounding the wire.

By way of further specific examples, molybdenum hexacarbonyl (orcarbonyl) is placed in dioxane and the dioxane heated at a temperatureof about 88 C., the hexacarbonyl readily enters soluiton and at 100 C.is completely in solution.

An iron substrate heated to 550 F. was exposed to vapors of the carbonyland dioxane while the dioxane was heated to the boiling point. Carbondioxide was passed through the solution and the iron substrate exposedto the vapors for a period of about 3 minutes. The deposit was hard,somewhat brittle, and provided a molybdenum coating on the substrate.

The above experiment was repeated utilizing tungsten carbonyl andmaintaining the same temperature conditions, and the resulting coatingwas hard, also somewhat brittle, and tungsten contained.

III

Experiment II was repeated except for the substitution of chromiumhexacarbonyl for the molybdenum hexacarbonyl. The resulting deposit,like the other resulting deposits, contained a metallic constituent andwas hard and slightly brittle.

Chrominum hexacarbonyl was dissolved in tetrahydrofuran having a boilingpoint of about 63 -64 C. At 55 C. the chromium hexacarbonyl enterssolution; in the present instance a saturated solution was formed asdescribed hereinbefore. The steel substrate was heated to a temperatureof 550 F. (287 C.) while the solution itself was heated to boiling withcarbon dioxide passed therethrough; the temperature of the solution wasindicated by thermometer in the solution to be about 170 F. C.).Exposition of the substrate to the vapors for a period of about threeminutes provided a heavy coat.

Each of the carbonyls, that is the hexacarbonyl of chromium, tungstenand molybdenum, dissolves in a tetrahydro furan. While the tungstencarbonyl appeared to enter solution more easily, the products tend to begray, but spectrographic analyses clearly indicate the presence ofchromium, tungsten and molybdenum in the various experiments.

While the invention has been specifically described in connection with1,4-dioxane and tetrahydro furan as the solvents for the carbonyls, ithas been contemplated that it may be possible to employ other ethers,such as 1,3-dioxane, or mixtures of ethers, such as 1,3-dioxane and1,4-dioxane. In'general 5 and 6 membered cyclic ether rings, whichexhibit stability at the temperature of decomposition of the carbonyl,and dissolve the carbonyl component, are considered useful in thepractice of the invention.

It is important to note that the pressure within the plating chamber maybe substantially atmospheric and that the exhaust at 9 (FIGURE 1) and at24 (FIGURE 2) may lead to the atmosphere.

Generally, since such gases contain in addition to the carbon dioxideand vaporized solvent some small amount of undecomposed carbonyl andsome carbon monoxides, it is well to provide for the combustion of theexhaust or for the recovery thereof.

Also, it is to be noted that cooling the coated material to preventoxidation in the atmosphere, while the coated material is highly heated,is frequently a desirable precaution against reaction of the atmosphere.

Also, the coating itself may be presented on the substate to theatmosphere to build up thicknesses where such is desired. Normally anexposure of three minutes to the vapors of a saturated solution issufiicient to produce a coating thickness of 0.2 to 0.3 mm.

It will be understood that this invention is susceptible to modificationin order to adapt it to diiferent usages and conditions and accordingly,it is desired to comprehend such modifications within this invention asmay fall within the scope of the appended claims.

What is claimed is:

1. In the process of gas plating metal on metal substrates whereindeposition of the metal is effected by thermal decomposition of acompound of said metal in the vapor state and while associated withvapors of a solvent for said compound, said process comprising the stepsof heating the substrate to the decomposition temperature of a metalcarbonyl selected from the group of molybdenum, tungsten and chromiumhexacarbonyls, dissolving the said metal carbonyl in dioxane and heatingthe resultant solution to form vapors of the metal carbonyl, heating thesubstrate to a temperature high enough to cause decomposition of thevapors of the metal carbonyl, passing an inert gas through the resultantheated metal carbonyl solution to entrain the carbonyl vapors andmaintain a saturated solution of the carbonyl, contacting the heatedsubstrate with said entrained carbonyl vapors while concurrently passingthe inert gas through the saturated solution of the metal carbonyl tofacilitate contact of the vapors of the metal carbonyl with thesubstrate whereby said vapors are thermally decomposed and the metalconstituent deposited on the substrate at a relatively high rate ofdeposition.

2. In the process of gas plating metal on metal substrates whereindeposition of the metal is effected by thermal decomposition of acompound of said metal in the vapor state and while associated withvapors of a solvent for said compound, said process comprising the stepsof heating the substrate to the decomposition temperature of a metalcarbonyl from the group of molybdenum, tungstem and chromium carbonyls,dissolving such a carbonyl in 1,4 dioxane, heating the resultantsolution in the presence of an excess of said metal carbonyl to providea saturated solution of the carbonyl and vapors of the metal carbonyl,passing an inert entraining gas through the resultant heated solution toentrain the vapors of the metal carbonyl and maintain a saturatedsolution of the carbonyl, and exposing the substrate while heated to atemperature to cause thermal decomposition of the metal carbonyl vaporand between about 200 and 315 F. 15 2,880,067

whereby the same is thermally decomposed and the metal constituentdeposited on the substrate.

3. As an article of manufacture, a gas plated article made in accordancewith the process of claim 1.

References Cited in the file of this patent UNITED STATES PATENTS2,375,482 Lyle May 8, 1945 2,477,554 McKeever July 26, 1949 2,638,423Davis et a1. May 12, 1953 2,785,651 Pawlyk Mar. 19, 1957 2,824,828 Homeret al. Feb. 25, 1958 2,867,546 MacNevin Jan. 6, 1959 Closson Mar. 31,1959

1. IN THE PROCESS OF GAS PLATING METAL ON METAL SUBSTRATES WHEREINDEPOSITION OF THE METAL IS EFFECTED BY THERMAL DECOMPOSITION OF ACOMPOUND OF SAID METAL IN THE VAPOR STATE AND WHILE ASSOCIATED WITHVAPORS OF A SOLVENT FOR SAID COMPOUND, SAID PROCESS COMPRISING THE STEPSOF HEATING THE SUBSTRATE TO THE DECOMPOSITION TEMPERATURE OF A METALCARBONYL SELECTED FROM THE GROUP OF MOLYBDENUM, TUNGSTEN AND CHROMIUMHEXACARBONYLS, DISSOLVING THE SAID METAL CARBONYL IN DIOXANE AND HEATINGTHE RESULTANT SOLUTION TO FORM VAPORS OF THE METAL CARBONYL, HEATING THESUBSTRATE TO A TEMPERATURE HIGH ENOUGH TO CAUSE DECOMPOSITION OF THEVAPORS OF THE METAL CARBONYL, PASSING IN INERT GAS THROUGH THE RESULTANTHEATED METAL CARBONYL SOLUTION TO ENTRAIN THE CARBONYL VAPORS ANDMAINTAIN A SATURATED SOLUTION OF THE CARBONYL CONTACTING THE HEATEDSUBSTRATE WITH SAID ENTRAINED CARBONYL VAPORS WHILE CONCURRENTLY PASSINGTHE INERT GAS THROUGH THE SATURATED SOLUTION OF THE METAL CARBONYL TOFACILITATE CONTACT OF THE VAPORS OF THE METAL CARBONYL WITH THESUBSTRATE WHEREBY SAID VAPORS ARE THERMALLY DECOMPOSED AND THE METALCONSTITUENT DEPOSITED ON THE SUBSTRATE AT A RELATIVELY HIGH RATE OFDEPOSITION.